The best Eevee evolution to choose in Pokemon Go is Umbreon, as it's a PvP champion that really excels in the Great League and the Ultra League.

What should I evolve my Eevee into?

Once you've walked 10km with that Eevee as your buddy Pokemon, it's ready to evolve into either Epseon or Umbreon. Just evolve in the day for Espeon and night for Umbreon - and this isn't limited, you can do it as often as you like!

Which Eevee evolution is best and strongest?

In terms of stats, Vaporeon has the highest HP among any of the Eevee evolutions. It is weak to grass and electric types, as expected for a water type Pokemon. It is strong against steel, fire, water, and ice Pokemon. At higher levels, it learns the moves Water Pulse, Aurora Beam, and Hydro Pump.